(1) Field of the Invention
The invention includes an apparatus and method of using acoustic waves (“signals”) to measure membrane fouling within a spiral wound membrane module. In one preferred embodiment, the invention includes a spiral wound module assembly including at least one membrane envelope wound about a permeate collection tube, an outer module housing, and an acoustic transducer located adjacent to the permeate collection tube. Acoustic signals are transmitted and received by the transducer and are compared with a reference signal to determine the state of membrane fouling.
(2) Description of the Related Art
Membrane fouling is a common problem in most membrane based separation processes. Membrane fouling is a chemical phenomenon where solutes are deposited upon the membrane surface resulting in reduced membrane flux and selectivity. Failure to timely clean membranes can result in higher cleaning costs (e.g. longer cleaning times, additional cleaning agents, use of more aggressive cleaning agents, etc.) and premature module replacement. Loss of operating time and increased costs associated with membrane cleaning and premature module replacement along with reduced operating performance result in overall increased separation costs. Thus, it is important to carefully monitor membrane fouling in order to optimize module performance, cleaning and longevity.
Spiral wound module configurations present specific challenges with respect to membrane fouling. Due to their spiral configuration, it is difficult to visually inspect the membrane surface without destroying or otherwise compromising the integrity of the module. Thus, membrane fouling is commonly monitored by a variety of indirect measures including: permeate flow rates, permeate recovery ratios, operating pressures, feed temperatures and permeate quality. Unfortunately, these indirect measures can be influenced by factors unrelated to membrane fouling, such as concentration polarization.
U.S. Pat. No. 6,161,435 describes a non-destructive, in-situ, real time, direct method for monitoring membrane fouling of spiral wound membrane module using Acoustic Time-Domain Reflectometry (ATDR), also referred to as UTDR (Ultrasonic Time-Domain Reflectometry). The module assembly includes acoustic piezoelectric transducers located on the outer housing of the module. Acoustic pulse signals are transmitted inward through the housing and into the spirally wound membrane envelopes. When the signal pulses encounter an interface, such as one formed between the feed solution and the top surface of the membrane envelope, a portion of the signal is reflected back to the transducer as an echo signal. The amplitude of the reflected signals depends on the acoustic impedance difference between the media or either side of the interface and the topography of the interface. The acoustic impedance is a function of the physical characteristics of the medium and is defined by the product of the density and acoustic signal velocity through the medium. Since the impedance, interface properties and path length change with an increase of fouling on the membrane surface, the change in amplitude, phase and the shift in arrival time of the interface echoes can be analyzed and used to monitor membrane fouling in real-time. Echo signals are compared via a signal processor with a reference signal (corresponding to an earlier measurement or a measurement from a database corresponding to a non-fouled membrane) so that the relative state of fouling or cleaning can be directly measured in real time.
Further descriptions of ultrasonic techniques for monitoring membrane fouling of spiral wound modules are provided in: Chai, G. Y., Greenberg, A. R., and Krantz W. B, In-situ Ultrasonic Measurements of Fouling and Cleaning Processes in Spiral-Wound Membrane Modules, Membrane Technology in Water and Wastewater Treatment 249, 266-257, (2000) Royal Society of Chemistry; Chai, G. Y., Greenberg, A. R., and Krantz W. B., Ultrasound, Gravimetric and SEM Studies of Inorganic Fouling in Spiral Wound Membrane Modules, Desalination 208, 277-293 (2007), Elsevier, Amsterdam; and Zhang, Zh.-X, Greenberg, A. R., Krantz, W. B.; and Chai, G. Y., Study of Membrane Fouling and Cleaning in Spiral Wound Modules Using Ultrasonic Time-Domain Reflectometry, New Insights into Membrane Science and Technology: Polymeric and Biofunctional Membranes, 65-88, (2003), A. A. Butterfield and D. Bhattacharyya, eds. Elsevier, Amsterdam. This latter reference indicates that the application of ultrasound to spiral wound modules is complicated by several factors including a much more complex signal pattern resulting from multiple reflections from the surface layers of multiple layers within spiral wound modules and loss of acoustic information caused by signal attenuation through these multiple layers as well as through the module housing. The reference goes on to describe a signal acquisition and analysis protocol which attempts to account for systematic shifts in the entire acoustic spectrum as a function of module operating time and enables information about the state of fouling to be obtained in real-time.
The outer housings of many commercially available spiral wound modules are made from fiber reinforced plastic, (e.g. glass fiber wrapped about wound membrane envelopes, coated or impregnated with a thermoplastic or thermoset resin such as an epoxy material). Unlike the relatively homogenous housing materials utilized with some modules, (e.g. stainless steel and polyvinylchloride), fiber reinforced plastics tend to scatter acoustic signals. That is, the combination of materials having distinct acoustic impedance properties along with many internal interfaces within an integral composite structure makes the interpretation of acoustic signals exceedingly difficult. As a consequence, the use of acoustic measurements through the outer housing of many spiral wound modules is quite limited. Moreover, unlike the aforementioned publications which focus on relatively small modules (e.g. 2.5 inch diameter), most industrial modules are much larger (e.g. 8 inch diameter and even larger). Due to their weight and dimension, these larger modules include much thicker outer housings. The use of relatively thicker fiber reinforced plastic housings further minimizes the utility of the aforementioned ultrasonic techniques.
Membrane fouling tends to initiate and be most pronounced in areas experiencing the highest permeate flux, i.e. areas of the membrane envelope adjacent to the permeate collection tube. Due to their spiral wound configuration, these susceptible areas of the membrane are wrapped within many concentric layers. That is, the most critical area of membrane surface for determining fouling is located at the most distant location from the outer housing of the module and is insulated by many concentric layers (e.g. membrane envelops, feed spacers, permeate spacers, etc.). Due to limitations of acoustic signal strength and the ever increasing complexity of interpreting reflective signals from multiple interfaces, the aforementioned ultrasound techniques are limited to examining only the outermost membrane layers. Unfortunately, these outer layers typically include tape and excess permeate and feed spacers as well as glued sections of membrane which are attached to the permeate spacer. Thus, the outer layers provide a less instructive measure of membrane fouling. That is, in order to take timely corrective action (e.g. cleaning, modification of feed quality, etc.), a measure of membrane fouling in the areas where fouling is initiated and/or most pronounced is desired.
In addition to monitoring membrane fouling, ultrasound has been described in a variety of other membrane applications including detecting membrane defects (U.S. Pat. No. 6,959,602), and membrane cleaning (U.S. Pat. Nos. 5,919,376 and 7,008,540)—including membrane cleaning from within spiral wound modules (U.S. Pat. No. 4,253,962).